BEIRUT, Lebanon — Insurgent commanders say that since Russia began air attacks in support of the Syrian government, they are receiving for the first time bountiful supplies of powerful American-made antitank missiles.

With the enhanced insurgent firepower and with Russia steadily raising the number of airstrikes against the government’s opponents, the Syrian conflict is edging closer to an all-out proxy war between the United States and Russia.

The increased levels of support have raised morale on both sides of the conflict, broadening war aims and hardening political positions, making a diplomatic settlement all the more unlikely.

The American-made TOW antitank missiles began arriving in the region in 2013, through a covert program run by the United States, Saudi Arabia and other allies to help certain C.I.A.-vetted insurgent groups battle the Syrian government.

The weapons are delivered to the field by American allies, but the United States approves their destination. That suggests that the newly steady battlefield supply has at least tacit American approval, now that Russian air power is backing President Bashar al-Assad.

“We get what we ask for in a very short time,” one commander, Ahmad al-Saud, said in an interview. He added that in just two days his group, Division 13, had destroyed seven armored vehicles and tanks with seven TOWs: “Seven out of seven.”

Spirits are rising on the government side as well. Weapons and morale are “at a new level,” said an official with the newly revived alliance of Russia, Iran and the Lebanese Shiite militia Hezbollah that is fighting on the behalf of Damascus.

Instead of a dim light at the end of a tunnel, the official said, speaking on the condition of anonymity to discuss military matters, the alliance is seeking something closer to victory. The aim now is to retake Syrian land that had been given up for lost, take the ouster of Mr. Assad off the table for good and reach a far more advantageous political solution after establishing “new facts on the ground.”

But as Russian airstrikes against Syrian insurgents have picked up, so have insurgent attacks, documented in online videos. TOW missiles weave across fields, their red contrails blazing, chasing Russian-made vehicles used by Syrian government forces and blowing them up.

At least 34 such videos have been posted in just the last five days from the battlefield in Hama and Idlib Provinces, where TOWs have helped blunt the Syrian government’s first ground offensive backed by Russian air power.

One official with a rebel group that is fighting in Hama called the supply “carte blanche.”

“We can get as much as we need and whenever we need them,” he said, asking not to be identified to avoid reprisals from rival Islamist insurgents he has criticized. “Just fill in the numbers.”

He said he believed Russia’s entry into the conflict had made the difference.

“By bombing us, Russia is bombing the 13 ‘Friends of Syria’ countries,” he said, referring to the group of the United States and its allies that called for the ouster of Mr. Assad after his crackdown on political protests in 2011.

The C.I.A. program that delivered the TOWs (an acronym for tube-launched, optically tracked, wire-guided missiles) is separate from — and significantly larger than — the failed $500 million Pentagon program that was canceled last week after it trained only a handful of fighters. That was unsuccessful largely because few recruits would agree to its goal of fighting only the militant Islamic State and not Mr. Assad.

Rebel commanders scoffed when asked about reports of the delivery of 500 TOWs from Saudi Arabia, saying it was an insignificant number compared with what is available. Saudi Arabia in 2013 ordered more than 13,000 of them. Given that American weapons contracts require disclosure of the “end user,” insurgents said they were being delivered with Washington’s approval.

Equally graphic videos of new Russian firepower have been posted by pro-government fighters and journalists embedded with them.

They appear to be using techniques honed in Afghanistan, where the occupying Soviet Army fought insurgents who were eventually supplied with antiaircraft missiles by the United States. Some of those insurgents later began Al Qaeda.

That specter hangs over American policy, and has kept Syrian insurgents from receiving what they most want: antiaircraft missiles to stop the government airstrikes that have been one of the war’s largest killers of civilians.

Now, they want them to use on Russian warplanes as well.

Mr. Saud, of Division 13, said he and other commanders renewed their requests for antiaircraft weapons 10 days ago to the liaison officers they work with in an operations center in Turkey.

“They told us they would deliver our requests to their countries,” he said. “We understand that it is not an easy decision to make when it comes to antiaircraft missiles or a no-fly zone, especially now that Syrian airspace is filled with jets from different countries.”

Both Russia and the United States have declared they are fighting the Islamic State, also known as ISIS or ISIL, but the two global powers support opposite sides in the battle between Mr. Assad and the Syrians who rebelled against his rule.

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Syrian soldiers and pro-government forces in the village of Atshan in Hama Province on Sunday.CreditSANA, via Agence France-Presse — Getty Images

With air support from Russia, the government of Mr. Assad is trying to retake territory seized this year in Idlib and Hama Provinces by insurgent groups that include both the Qaeda-affiliated Nusra Front and American-backed units calling themselves the Free Syrian Army — but not ISIS, which is strong in northern and eastern Syria into Iraq but has little presence in the west.

Instead, the advances there, which have posed the most immediate threat to Mr. Assad, have come from a coalition of Islamist insurgents called the Army of Conquest, which includes the Nusra Front but opposes the Islamic State.

Advancing alongside the Islamist groups, and sometimes aiding them, have been several of the relatively secular groups, like the Free Syrian Army, which have gained new prominence and status because of their access to the TOWs.

Even in smaller quantities, the missiles played a major role in the insurgent advances that eventually endangered Mr. Assad’s rule. While that would seem like a welcome development for United States policy makers, in practice it presented another quandary, given that the Nusra Front was among the groups benefiting from the enhanced firepower.

It is a tactical alliance that Free Syrian Army commanders describe as an uncomfortable marriage of necessity, because they cannot operate without the consent of the larger and stronger Nusra Front. But Mr. Assad and his allies cite the arrangement as proof that there is little difference between insurgent groups, calling them all terrorists that are legitimate targets.

Either way, the newly empowered Free Syrian Army, long a marginal player as Islamist groups have risen in influence, is playing a more prominent role.

“Islamic groups have always labeled us as agents, infidels and apostates because of our dealing with the West,” Mr. Saud said. “But now they can see how effective we are because of our dealing with the West.”

Several American-aided units have come under direct fire by the Russians. But they claim to have held their territory, with the help of TOW missiles, better than their Islamist counterparts.

In a further shift of American aid to fighting groups already operating inside Syria, American cargo planes on Sunday dropped the first shipment of small-arms ammunition to Syrian Arab fighters combating the Islamic State, a military spokesman, Col. Steve Warren, said on Monday.

He declined to identify the groups or their locations, citing operational security, but said American officials had screened them. The likely recipient was a coalition of mixed Arab and Kurdish groups that have been battling Islamic State fighters in northeastern Syria alongside Kurdish militias, now calling itself the Syrian Arab Coalition.

Syrian government troops advanced on Monday toward a strategically important highway held by insurgents, taking several villages in the central province of Hama with the help of Russian airstrikes, according to Syrian and Russian state news media, antigovernment activists and fighters.

But the front lines remained heavily contested, according to activists, with each side making liberal use of its new weapons.